In recent years, research and development of a light-emitting element (organic EL element) which uses an organic compound and utilizes electroluminescence (EL) have been actively promoted. In the basic structure of such a light-emitting element, an organic compound layer containing a light-emitting substance (an EL layer) is interposed between a pair of electrodes. By applying voltage to this element, light emission from the light-emitting substance can be obtained.
Since such a light-emitting element is of self-light-emitting type, it is considered that the light-emitting element has advantages over a liquid crystal display in that visibility of pixels is high, backlight is not required, and so on and is therefore suitable as flat panel display elements. Besides, displays including such light-emitting elements have advantages in that it can be formed to be thin and lightweight, and has quite fast response speed.
In such a light-emitting element, light-emitting layers can be successively formed two-dimensionally, so that planar light-emission surface can be obtained. Therefore, large-area elements can be easily formed. This is a feature which is difficult to be obtained by point light sources typified by an incandescent lamp and an LED or linear light sources typified by a fluorescent lamp. Accordingly, the light-emitting element is extremely effective for use as a surface light source applicable to illumination and the like.
As described above, an organic EL element has various features; however, price is the biggest barrier to be overcome for diffusion.
The price of liquid crystal displays, which is a typical of flat panel displays, has been continuously declining and also the price of LED lightings is gradually declining. As long as the price difference between organic EL elements and these preceding products is too large, the diffusion of organic EL devices is not promoted even though they have many advantages other products do not have.
Additionally, the basic performance such as lifetime, power consumption, and the like have to be equivalent to or better than other competing products than the problem of price.
Patent Document 1 focuses on a halide contained in an organic compound included in an EL layer and discloses a light-emitting element with long lifetime obtained by limiting the concentration of halides to a certain level.